The force exerted on a spherical particles of magnetisable material in a magnetic field is given by the expression: ##EQU1## where Xm is the volume magnetic susceptibility of the material, D is the diameter of the particles, H is the magnetic field intensity and dH/dx is the rate of change of the magnetic field intensity with distance. From this expression it can be seen that, if both the diameter D and the volume magnetic susceptibility Xm of a particle are small, it is necessary to provide a high intensity magnetic field and/or a magnetic field whose intensity changes rapidly with distance to exert an appreciable force on the particle.
It can be shown that, in a simple wet magnetic separator containing magnetisable material constituted by a single ferromagnetic wire of radius a and saturation magnetisation M.sub.s, the chance of a paramagnetic particle, of radius R and magnetic susceptibility Xm in a fluid of viscosity .eta. moving with velocity V.sub.o relative to the wire in a uniform magnetic field of intensity H applied in a direction opposite to the direction of flow of the fluid, being captured by the wire, whose longitudinal axis is orientated in a direction perpendicular to the direction of the magnetic field and to the direction of flow of the fluid, increases with the ratio V.sub.m /V.sub.o, where V.sub.m is a quantity having the dimension of speed which may be called the "magnetic velocity" and is given by the expression: ##EQU2##
It can therefore be seen that the chance of capturing the paramagnetic particle may be increased either by increasing the value of the magnetic field intensity H or by decreasing the value of the velocity V.sub.o.
Considering both expressions (1) and (2) together, it can be seen that a high efficiency of capture of magnetisable particles in a magnetic separator may be achieved either by maximising the value of the force F in expression (1), or alternatively by reducing the value of the velocity V.sub.o in which case the magnetisable particle may be acted upon by a smaller force but passes the collecting sites within the packing material at a lower velocity and therefore has a greater chance of being captured. However, if the value of V.sub.o is small, the amount of feed material which can be passed through the separating chamber in a given time will be small.
Canadian patent specification No. 937,537 discloses a continuous magnetic separator comprising a plurality of separating chambers linked together to form an endless loop. The loop of separating chambers is continuously rotatable between pairs of yokes of electromagnets. Each separating chamber has an open top and a perforated bottom and contains magnetisable material in the form of partially cut and expanded sheet metal. In use of the separator, the material to be treated is introduced into each separating chamber when that separating chamber is within a magnetic field associated with one of the electromagnets. Magnetisable particles within the feed material are retained in the magnetisable material while the remainder of the feed material passes straight through the separating chamber and out through the perforated bottom. Whilst, in use of such a separator, a relatively high throughflow rate of feed material may be provided whilst maintaining a reasonable efficiency of separation (by the choice of a suitably low value of V.sub.o), the velocity V.sub.o which is related to the throughflow cross-section of the separating chambers will still be such that the more weakly magnetisable particles will not be separated (unless a very high intensity magnetic field is utilized). Also the flow of feed material through the separating chambers will be turbulent, thus providing varying values of V.sub.o throughout the volume of each separating chamber and prejudicing the separation efficiency. Thus the most weakly magnetisable particles either will not be captured within the magnetisable material or will be captured and subsequently swept out of the material.